Read Only Memory - ROM
ROM is memory containing hardwired instructions that the
computer uses when it boots up, before the system software loads. In PCs, the
instructions are read from a small program in the ROM, called the BIOS (Basic
Input/Output System).
ROM is "built-in" computer memory
containing data that normally can only be read, not written to. ROM contains
the programming that allows your computer to be "booted up" or
regenerated each time you turn it on. Unlike a computer's Random Access Memory (RAM),
the data in ROM is not lost when the computer power is turned off. The ROM is
sustained by a small long-life battery in your computer. If you ever do the hardware setup procedure
with your computer, you effectively will be writing to ROM.
Random
Access Memory - RAM
Pronounced `RAMM´, acronym for Random
Access Memory, a type of computer memory that can be accessed
randomly; that is, any byte of memory can be accessed without touching the
preceding bytes. RAM is the most common type of memory found in computers and
other devices, such as printers.
`There are two
basic types of RAM´:
Dynamic RAM (DRAM)
Static RAM (SRAM).
The two types differ in the technology they use to hold
data, dynamic RAM being the more common type. Dynamic RAM needs to be refreshed
thousands of times per second. Static RAM does not need to be refreshed, which
makes it faster; but it is also more expensive than dynamic RAM. Both types of
RAM are volatile, meaning that they lose their contents when the power
is turned off.
In common usage, the term RAM is synonymous with main memory the memory available to
programs. For example, a computer with 8M RAM has approximately 8 million bytes
of memory that programs can use. In contrast, ROM (Read Only Memory) refers to
special memory used to store programs that boot the computer and perform
diagnostics. Most personal computers have a small amount of ROM (a few thousand
bytes). In fact, both types of memory (ROM and RAM) allow random access to be
precise; therefore, RAM should be referred to as read/write RAM and ROM as read only.
RAM (Random Access Memory) is the place in a computer where
the operating system application programs and data in current use are kept so
that they can be quickly reached by the computer's processor. RAM is much
faster to read from and write to than the other kinds of storage in a computer,
the hard disk, floppy disk, and CD-ROM. However, the data in RAM stays there only
as long as your computer is running. When you turn the computer off, RAM loses
its data. When you turn your computer on again, your operating system and other
files are once again loaded into RAM, usually from your hard disk.
RAM can be compared to a person's short-term memory and the
hard disk to the long-term memory. The short-term memory focuses on work at
hand, but can only keep so many facts in view at one time.
If short-term memory fills up, your brain sometimes is able
to refresh it from facts stored in long-term memory. A computer also works this
way. If RAM fills up, the processor needs to continually go to the hard disk to
overlay old data in RAM with new, slowing down the computer's operation. Unlike
the hard disk which can become completely full of data so that it won't accept
any more. RAM never runs out of memory.
It keeps operating, but much more slowly than you may want it to.
RAM is small, both in physical size (it's
stored in microchips) and in the amount of data it can hold. It's much smaller
than your hard disk. A typical computer may come with 256 million bytes of RAM
and a hard disk that can hold 40 billion bytes. RAM comes in the form of
"discrete" (meaning separate) microchips and also in the form of
modules that plug into holes in the computer's motherboard. These holes connect
through a bus or set of electrical paths to the processor. The hard drive, on the other hand, stores
data on a magnetized surface that looks like a phonograph record.
Most personal computers are designed to allow you to add
additional RAM modules up to a certain limit. Having more RAM in your computer
reduces the number of times that the computer processor has to read data in
from your hard disk an operation that takes much longer than reading data from
RAM. (RAM access time is in nanoseconds; hard disk access time is in
milliseconds.)
RAM is called "random access"
because any storage location can be accessed directly. Originally, the term
distinguished regular core memory from offline memory, usually on magnetic tape
in which an item of data could only be accessed by starting from the beginning
of the tape and finding an address sequentially. Perhaps it should have been
called "nonsequential memory" because RAM access is hardly random.
RAM is organized and controlled in a way that enables data to be stored and
retrieved directly to specific locations. A term IBM has preferred is direct
access storage or memory. Note that other forms of storage such as the hard
disk and CD-ROM are also accessed directly (or "randomly") but the
term random access is not applied to these forms of storage.
In addition to disk, floppy disk, and CD-ROM storage,
another important form of storage is read-only memory (ROM), a more expensive
kind of memory that retains data even when the computer is turned off. Every
computer comes with a small amount of ROM that holds just enough programming so
that the operating system can be loaded into RAM each time the computer is
turned on.
